1. Field of the INVENTION
This invention relates, in general, to power distribution line communication systems and, more specifically, to signal repeaters for communication systems associated with power distribution lines.
2. Description of the Prior Art
Communication repeaters are used in power distribution line systems to relay communication signals between various points in the distribution system. Uses for such repeaters have included automatic meter reading applications and power line switching applications. Generally, the repeaters are located at positions on the distribution system where the strength of the communication signal is becoming weak and needs to be amplified to maintain a sufficient signal-to-noise ratio. Repeaters are also used at locations where power line components exhibiting a high attenuation for communication signals are connected to the power line system, such as phase angle correcting capacitors and distribution transformers.
Conventional repeaters usually include at least one receiver which detects signals on the distribution line and one transmitter which retransmits these signals to the distribution line after proper amplification. Repeaters are also used in various operating modes, with one of the most frequently used being the half-duplex mode. A half-duplex system permits the transmission of communication signals in both directions along the distribution line system, but only in one direction at a time. Repeaters operating in the half-duplex mode have typically included two transmitters and two receivers, all of which are coupled to the power distribution line. In many applications, the communication signal is translated from one frequency to another. That is, the signal is received at one frequency and translated to another frequency before being applied again to the distribution line.
Repeaters operating in the half-duplex mode are usually controlled by circuitry which senses the reception or detection of a signal having a specific frequency. When the receiver is thus activated, the corresponding transmitter is turned on to retransmit the received signal along the distribution line. Since both transmitters and both receivers are located relatively close to each other on the power distribution line, much of the signal transmitted by one transmitter is necessarily applied to the receiver associated with the other transmitter. Therefore, large and sophisticated filtering networks are needed to filter out the unwanted signals from the operating transmitter in order to prevent the receiver associated with the non-operating transmitter from being activated. Even with large filtering components used in such a system, intermittent operation has been observed due to the detection by both receivers simultaneously of a sufficient signal to activate their associated transmitters. Therefore, it is desirable, and it is an object of this invention, to provide a communication repeater for use on a power distribution line which conveniently and economically prevents the activation of two high power amplifier stages at the same time.